Consider a solar cell (SC) wherein most of the front surface (FS)—the solar cell side receiving sunlight, also called the sunny surface or sunnyside—is NOT directly connected to an emitter or base. In one known SC design, the SC has contacts and wires for emitters and bases near or on the back surface (BS)—also called the shady surface, non-sunnyside, or backside. In another known solar cell design, the solar cell has the base contact on the back surface and emitter contacts that penetrate (“wraps through”) the solar cells.
Solar cells often suffer from wasted energy near the front surface. For short wavelength light on a crystalline silicon solar cell, light absorption and generation of electrons and holes (electron-hole pairs) occurs very close to the front surface. In this region, if there are any electronic traps or recombination centers, the energy in electrons or holes may be absorbed and degraded into heat and thus wasted. Depending on details of solar cell fabrication and passivation, it may be difficult to sufficiently minimize such traps and recombination centers.
Solar cells also often suffer from uneven illumination. Consider a number of solar cells connected in series and one solar cell is shaded but the other connected solar cells are brightly illuminated. In this case, the illuminated solar cells will generate much more power than the shaded solar cell. Therefore, power is generated by the illuminated solar cells and is dissipated in the shaded solar cell. This may cause the shaded SC to become very hot and suffer permanent damage. One solution to this problem is to provide a bypass diode in parallel with a solar cell or in parallel with a series string of solar cells.
Known solutions for solar cell front surface degradation include a solar cell with a bias voltage or charge on a transparent conductive layer (or “gate”) on the sunny surface of the solar cell to minimize recombination. Another known solution includes a gate on the sunny surface of the solar cell to minimize recombination.
Known solutions to the problems above, front surface degradation and unequal illumination, are very separate. In other words, the solutions for one problem do NOT provide solutions to the other.